1. Field of the Invention
The present invention relates to a photodetector in which a plurality of photodiodes are disposed.
2. Description of the Background Art
With the increased interest in image sensors, image disturbance caused by light crosstalk between photodiodes (straying of light incident on a photodiode into adjacent photodiodes) is giving rise to problems (refer to Masatoshi Ishihara et al., “InGaAs Linear Image Sensor for Near-Infrared Range and their applications”, Light Application and Visual Science, Institute of Electrical Engineering of Japan, JN: Z0953A, Vol. LAV-00, No. 7-13, pp. 31-36 (2006/10/26); Non-patent Reference 1). As shown in FIG. 23, in a photodetector 150, a p-electrode 107 of each photodiode and an n-electrode 106 are electrically connected to a multiplexer 135 via In solder 134. In this structure, crosstalk corresponds to the following phenomenon. As shown in FIG. 24, light incident on a photodiode 110 is received by a light-receiving portion 116 and converted into a current. A part of the light passes through the light-receiving portion 116 and is reflected by a p-electrode 107 located in the center of the mounting surface side of the photodiode and is also received by the light-receiving region of an adjacent photodiode. In the photodetector 150 shown in FIG. 24, photodiodes 110 are mounted epitaxial-side-down. The crosstalk phenomenon occurs in the same manner when photodiodes 110 are mounted epitaxial-side-up. In the photodetector, an anti-reflection film (AR film) 113 is disposed on the top surface of an InP substrate 151, and on the bottom side, i.e., on the mounting side, a structure of n-type buffer layer 102/absorption layer 103/InP window layer 104/p-electrode 107 and SiN passivation film 112 is disposed. FIG. 25 shows a photodetector 150 provided with photodiodes 110 epitaxial-side-up mounted. In this photodetector, crosstalk also occurs, in which light received by a photodiode 110 is also received by adjacent photodiodes. As a result of such crosstalk, a signal current is generated between adjacent photodiodes (pixels), and resolution is degraded. With respect to the photodetector 150 shown in FIG. 24, description will be made in detail below in DETAILED DESCRIPTION OF THE INVENTION. In the photodetector described above, as the pitch of the arrayed photodiodes decreases, degradation in resolution becomes more significant. Consequently, it is not possible to increase the number of pixels in a sensor having a certain size. When the number of pixels is increased, it is necessary to increase the chip size, which results in an increase in cost and which runs counter to miniaturization.
In order to reduce light crosstalk between pixels, a structure has been proposed in which a separation groove is provided between adjacent photodiodes and a metal film is disposed therein to block crosstalk between the adjacent photodiodes (refer to Japanese Unexamined Patent Application Publication No. 2005-123217, Patent Reference 1). Furthermore, although not directly related to crosstalk, a proposal has been made in which, in a near infrared photodiode array structure, photodiodes are disposed in an integrated manner on an integrated circuit to increase the light-receiving area, and thus to increase sensitivity (refer to Japanese Unexamined Patent Application Publication No. 11-354762, Patent Reference 2).
In the structure in which the separation groove and the metal film are provided between adjacent photodiodes, although light crosstalk can be reduced to a certain degree, since light propagates through the semiconductor substrate freely, light subjected to multiple reflections through the semiconductor substrate strays into adjacent photodiodes. Consequently, it is not possible to suppress crosstalk sufficiently. Furthermore, in the structure described above, since an insulation film is provided between photodiodes and a metal film is disposed thereon, consistency of quality required for the insulation film (reliability requirement) is significantly high. Therefore, for example, the presence of even a single pinhole or the like in the insulation film causes electrical short, thus causing a failure in function as a photodiode. Furthermore, the decrease in yield and the increase in cost are not negligible. In the structure in which photodiodes are disposed in an integrated manner on an integrated circuit, although the photodiodes are isolated from each other, free passage of scattered light is allowed. Thus, the problem of crosstalk is not solved.
If the crosstalk occurs, resolution between pixels is degraded. As the pitch between photodiodes is decreased, crosstalk increases significantly, and it becomes difficult to dispose pixels within a limited area. Furthermore, if the pitch between photodiodes is increased and the number of pixels is increased, the chip size increases as described above, resulting in an increase in cost.